Stored program controlled telephone switching systems are used to interconnect telephone station sets as well as digital terminals, personal computers and large main-frame computers. The telephone switching system establishes communication connections between these computer facilities in a manner analogous to voice communications connections between subscribers who are using analog telephone station sets. The computer facilities are connected to communication pairs of the telephone switching system by modems. A standard telephone station set is also connected to the communication pair and is used to originate a call to a destination computer facility. As a result of the call origination, a communication connection is established through the switching network from the originating telephone station set to the destination computer facility. The user then switches the modem on line and the modem converts the digital signals output by the computer facility to analog signals which are transmitted by the switching network to a modem associated with the destination computer facility. The destination modem converts the received analog signals to digital signals for use of the destination computer facility. Thus, the telephone switching system simply provides a communication path between two designated end points which, in this case, are connected to computer facilities.
The stored program controlled telephone switching system uses computers to govern its operation. A system processor or a number of system processors operating in synchronization are hard-wired into the telephone switching system to control call establishment and switching network operation. Telephone switching systems also make frequent use of hard-wired micro-processors to perform low level time consuming tasks such as line scanning, protocol conversion, etc. thereby freeing the system processor to implement the high level call processing routines.
There is a clear separation between these two computer environments. In one situation, computers are hard-wired into the telephone switching system to realize fixed telephone switching system control functions while in the other situation the telephone switching system interconnects customer owned computer facilities in a fashion analogous to the interconnection of analog station sets.